Homunculi are creatures created through Alchemy. A Homunculus can be large or small and come in many shapes when fully formed, though Homunculi always start off very small and transparent when first created, and grow to full size with time.
The time it takes a Homunculus to grow varies based on the ingredients used in the potion which created it, but the Homunculus' lifespan is always determined by it's growth rate, a very rapidly-growing Homunculus will die within days, hours, or even minutes of it's birth, while a Homunculus with a human-like growth rate will live for many decades.
Homunculi, despite their seemingly magical nature, are completely biological beings that require rest and sustenance to survive. Homunculi are typically fed a diet of blood and other liquids, though some Homunculi are intentionally designed to be carnivorous and feast off of their foes.